A vehicle often has an ultrasonic system for ascertaining a position of an object in an area surrounding the vehicle. Such an ultrasonic system is described in German Patent No. DE 38 130 83 A1. An ultrasonic system of that kind has at least one transmitting device and one receiving device mounted on the vehicle. In particular, the ultrasonic system may also include a plurality of transmitting and receiving devices distributed over the vehicle. The transmitting and receiving devices are normally built into a unit and use one and the same electroacoustic transducer, the diaphragm, as a bidirectionally actuated antenna device. In this case, the antenna device may be used both as transmitter and as receiver for an ultrasonic signal. Frequently, actuation of an ultrasonic system having a plurality of antenna devices takes place in such a manner that only few antenna devices emit an ultrasonic signal as the transmission signal at any one time. To avoid detection ambiguities, after emission of what is usually a pulse-form transmission signal, there is a waiting period, referred to as an echo cycle, before a next transmission pulse is emitted. The period of time required until all antenna devices of the ultrasonic system have sent a transmission signal at least once is frequently referred to as the transmission cycle of the ultrasonic system.
A transmission signal emitted by an antenna device is able to impinge on at least one reflecting object point and be reflected at least partially at that object point. An antenna device of the receiver then receives the reflected ultrasonic signal and ascertains the length of time between emission of the transmission signal and receiving of the reflected ultrasonic signal. An evaluation device of the ultrasonic system is then able to determine, on the basis of the at least one length of time ascertained, a position of the at least one reflecting object point in an area surrounding the vehicle.